French philosopher who thought SS created mayhem (9)
I believe the answer is:
descartes
'french philosopher who thought' is the definition.
(Rene Descartes, who said 'I think therefore I am')
'ss created mayhem' is the wordplay.
'mayhem' indicates an anagram.
'ss'+'created'='sscreated'
'sscreated' anagrammed gives 'DESCARTES'.
(Other definitions for descartes that I've seen before include "One who thought" , "Cogito ergo sum philosopher" , "Cogito, ergo sum man" , "French mathematician and philosopher" , "Rene --, philosopher" .)